The standard technique in the art for measuring an optical absorption coefficient α of a liquid uses an optical cell (a cuvette) in which a collimated beam of light goes through the liquid along a path of well-defined length l. The cell is filled with a reference liquid, and the light detected I1 through the cell containing a reference liquid of coefficient α1 is measured. Then the cell is filled with the unknown liquid of unknown coefficient α2, and the intensity I2 through the same cell is measured. The coefficient α2 is then obtained by the expression:α2=α1+{loge I1−loge I2}/l 
This method has several drawbacks. It requires the use of collimated light and will not work accurately with non-collimated light. Furthermore, the Beer's Law equation from which the expression above derives assumes ideal conditions: it does not take account of reflections introduced by the four interface surfaces traversed by the light (air-glass followed by glass-liquid at the front glass wall of the cell, and then liquid-glass followed by glass-air at the back glass wall). Multiple reflections can occur which means that the pathlength assumption is incorrect in practice. If liquids of different refractive index are employed then the amount of light reflected at the liquid-glass interfaces changes between the reference liquid and the unknown liquid. The method also assumes that all of the absorption occurs due to the liquid and none due to the glass.
Turbidity is typically measured using a commercial turbidimeter which measures the transmission of light through a standard cuvette in similar manner and with similar drawbacks.